Field of the Invention
The present invention relates to a wireless power supply technique.
Description of the Related Art
In order to charge an electric shaver, an electric toothbrush, a cordless phone, a game machine controller, an electric power tool, or the like, contactless power transmission (which is also referred to as “contactless power supply” or “wireless power supply”) is used. FIG. 1 is a diagram showing a configuration of an electronic device including a wireless power receiving apparatus investigated by the present inventors.
An electronic device 1r includes a wireless power receiving apparatus 300, a secondary battery 2, a DC/DC converter 4, and a micro-controller (MCU: Micro-Controller Unit) 6. The wireless power receiving apparatus 300 receives an electric power signal S1 from a wireless power supply apparatus 200, and charges the secondary battery 2. For example, the secondary battery 2 is configured as a nickel hydride battery or a lithium-ion battery. The DC/DC converter 4 steps up or otherwise steps down the voltage VBAT of the secondary battery 2 so as to supply a power supply voltage to the micro-controller 6. The micro-controller 6 controls the overall operation of the electronic device 1r. 
The wireless power supply apparatus 200 supplies an electric power signal to the wireless power receiving apparatus 300. The wireless power supply apparatus 200 includes a transmission coil 202 and a driver 204. The driver 204 is configured as a voltage source or otherwise a current source, and applies an AC driving current to the transmission coil 202.
The wireless power receiving apparatus 300 includes a reception coil 302 arranged in the vicinity of the transmission coil 202 such that they are coupled with each other. When the driving current flows through the transmission coil 202, electromagnetic induction occurs, thereby supplying a coil current ICOIL that flows through the reception coil 302.
The wireless power receiving apparatus 300 includes a diode bridge circuit 304, an output switch 306, a control switch 308, and resistors R10 through R12 in addition to the reception coil 302. With conventional techniques, the wireless power receiving apparatus 300 is configured as a combination of so-called discrete components.
The diode bridge circuit 304 includes four bridge-connected diodes. The diode bridge circuit 304 rectifies the coil current ICOIL that flows through the reception coil 302, so as to generate a charging current ICHG. The resistor R10 is connected to the output terminal of the diode bridge circuit 304. The charging current ICHG is supplied to the secondary battery 2 via the output switch 306, thereby charging the secondary battery 2.
The wireless power receiving apparatus 300 receives, as an input signal, a control signal CTRL generated by the micro-controller 6 as an instruction to supply electric power to the secondary battery 2. When the control signal CTRL is asserted (set to high level), the output switch 306 is turned on. In this state, the charging current ICHG is supplied to the secondary battery 2. Examples of such a secondary battery 2 include a nickel hydride battery, a lithium-ion battery, and the like. The output switch 306 is configured as a PNP bipolar transistor. The resistor R12 and the control switch 308 are arranged between the control terminal (base) of the output switch 306 and the ground line. The control switch 308 is configured as an NPN bipolar transistor. The control signal CTRL is input to the base of the control switch 308. Furthermore, the base of the control switch 308 is connected to the output of the diode bridge circuit 304 via the resistor R11.
When the control signal CTRL is set to high level, the control switch 308 is turned on, which supplies the ground voltage to the base of the output switch 306. As a result, the output switch 306 is turned on, which supplies the charging current ICHG to the secondary battery 2. When the control signal CTRL is set to low level, the control switch 308 is turned off, which suspends the charging operation.
The wireless power receiving apparatus 300 according to a conventional technique is configured employing discrete components. As a result, there is a need to employ the diode bridge circuit 304 as a rectifier circuit. This leads to large power loss due to the diodes, resulting in reduced efficiency of the wireless power receiving apparatus 300.